As a motor vehicle is being driven, it is not uncommon for one of the driven wheels to lose its grip on the road. To promote safety, traction control systems have been developed for detecting wheel spin and reducing wheel torque in response to wheel spin. In one traction control system, described in U.S. Pat. No. 6,182,002 B1, vehicle acceleration is measured and used to detect wheel spin. Another traction control system is described in U.S. patent application Ser. No. 10/601,063, filed Jun. 20, 2003, entitled “Wheel Slip Detection and Torque Management”, which is commonly assigned and hereby incorporated by reference in its entirety. This traction control system uses pulley speed of a transmission, such as a continuously variable transmission (CVT), to detect wheel slippage.
In other systems, wheel speeds are monitored to detect wheel spin. For example, in vehicles having anti-lock brake systems (ABS), wheel speed sensors are used to provide information to a traction control system. As a result, wheel slippage can be detected directly.
Speed sensors also play other important roles in a motor vehicle. For example, a CVT ratio is determined as a function of throttle pedal signal, pulley speed signal, and torque converter turbine speed signal. As a result, various output speeds are combined to affect control of the vehicle. Further, some output speeds, such as pulley speed, may play multiple roles.
A problem therefore arises in each of the forgoing systems and methods when a speed sensor exhibits instability resulting, for example, from a missing tooth or electrical instability. For example, when instability occurs in the output pulley speed or turbine speed, the ratio of the CVT is perturbed. Also, when instability occurs in output pulley speed or output wheel speed, engine flare may result due to incorrect detection of wheel slippage.